Diagenesis Of Altered Volcaniclastic Rocks Of The Permian In The Northwestern Margin Of Junggar Basin

The microscopic study and analysis of the Permian volcaniclastic rocks samples from 20drilled wells in the northwestern margin of Junggar basin shows that authigenic minerals identification and distribution features,diagenetic evolution,and diagenetic conditions have an influence on reservoir quality.Permian volcaniclastic rocks are dominantly sandy-conglomerate and conglomerate attributed to tuff clasts.Detrital grains have observed as rounded to subrounded and angular to subangular in shape.The central diagenetic authigenic mineral cement is a zeolite,clay minerals,albite,and calcite.Analcime,laumontite and heulandite zeolites cement are the most common minerals in the study area.Zeolites and additional authigenic minerals were formed during diagenesis is an open hydrologic system.Typically volcanic glass transformed from both clay minerals and zeolite minerals with respect to distribution characteristics can be constructed from same aluminosilicate precursor materials.Alterations of these minerals during diagenesis are in the order such as volcanic glass-clinoptilolite-analcite-heulandite-laumontite zeolite minerals and at the same time volcanic glass-smectite-chlorite clay minerals.At the eodiagenetic stage,representative diagenesis included compaction,hydration,devitrification,and smectite formation.During mesodiagenetic stage representative,diageneses included cementation,dissolution as well as the replacement.Groundwater chemistry,p H of pore water and the ratio of cation exchange have directly influenced the reservoir quality.Dense grains are compacted and can be identified by the line and concave-convex grain contacts.Chlorite coated grains and cement dissolution can contribute to good porosity.In contrast to compaction,calcite and heulandite cement are the dominant factors for the formation of low porosity reservoir by filling intergranular pores.The dissolution of analcime and laumontite cement can be observed in the study area;chlorite coating preserved the pores from compaction.CO₂ and acidic organic rich water is the source of dissolution.This study reveals that,the dissolution occurred in the whole mesodiagenetic stage has created secondary porosity which enhances the reservoir quality.